I've said this before, but for me its just simple logic: Its 2019, the first 2019 TVs already ship with HDMI 2.1, so by 2020 its going to be almost all of them. I can wait another year, and every year that I delay an upgrade also increases my theoretical budget. Some people certainly upgrade TVs a lot more often then me though, and thats fine.
And even if I don't buy a HDMI 2.1 AVR immediately, I certainly will within the next couple years. That said, I'll likely do it immediately, since my old one is pre-Atmos/DTS:X. But good AVRs cost more then TVs, its ridicoulous, if you add decent speakers to that (good, but not outrageous), you can almost buy 2 TVs. Luckily good speakers last you a lifetime. Just need to get the Atmos Height-Speaker extension.
Thinking about it, it somehow seems silly to try to aim for the best TV possible and even wait years and years to get one, considering its in reality not *that* expensive if you compare it to other things. High-end phones cost as much as a good 55" TV. My PC setup probably costs as much as a few of those.
My main concern is where I would put all the old ones..